i have a mec 600 jr shot shell re loader. at the top it has a red side bar for dropping powder and shot.?

i was wondering since im reloading 12 gage 7 and 1/2 winchester 2 3/4 inch shells if at the top where that slide is that measures the powder if i need to buy a coupling to fit in those to measure exact powder and shot.

I highly suggest you look up shotshell reloading in a manual or at least from a reliable online source.

You ABSOLUTELY need a powder bushing to fit inside the powder side of the charge bar. Which number depends on the powder you are using. You can use a MEC powder bushing chart to get an idea, but you NEED a scale. I say again, YOU NEED A SCALE to weigh the powder charge you're dropping in grains.

DO NOT listen to "700" saying shotguns aren't "too testy". You need to get a bushing or two that drop around the amount of powder you want to drop per the chart. Then set up the loader and before you load any shells at all, pour the charge out and weigh it, several times over (use a shell with a dead primer and dump the powder and shot back in the tubes when done). Your drops will vary 1-2 TENTHS (0.1) of a grain - this is OK, but you need to know how much you're putting in on average, per the manual. NEVER go above the MAX or below the starting load - anywhere in between is fine.

The bushing determines the powder, however the bar itself determines the shot. There should be two different numbers on the end...a number beginning with 3...and another that will be 78, 100, 118, 114, etc.. 78 is 7/8oz of shot, 100 is 1oz, 118 is 1 1/8oz, etc..

Make sure you are using the right shell, wad, primer, amount of shot, and exact powder, per published reloading data. ONLY use exact data and follow it religiously - it's imperative for reloading safely.